In the field of optical communications, research has been directed to the development of diode laser transmitters for use within fiber optic communications networks. Techniques have been devised for modulating both the intensity and the emission wavelength of the diode laser by varying the applied current. For example, digital coding of an optical input signal has been achieved by alternately turning the laser diode current on and off. However, such current modulation techniques may be unable to provide the modulation speed required for high capacity fiber optic networks.
As a consequence, other techniques of high-speed optical modulation relying upon modulators positioned in the path of optical emission from the laser source have been investigated. Unfortunately, the size and temperature sensitivity of such external modulators may render these devices inappropriate for certain fiber optic network applications.
In another approach, the emission frequency of a semiconductor laser is acoustically modulated using an ultrasonic wave. Frequency modulation via ultrasonic waves arises due to the pressure dependence of the dielectric constant of the laser active layer. For small pressures, such as are produced by ultrasonic waves, the shift in the spontaneous emission line is negligible and the behavior of the laser modes is governed entirely by the acoustical modulation of the dielectric constant. If alternating pressure is applied to the laser via ultrasonic waves, corresponding frequency modulation of the optical output is observed.
This type of acoustical modulation technique has been effected through an apparatus in which a quartz transducer is bonded to one side of a semiconductor injection laser, with the other side of the laser typically being bonded to a heat sink. See, for example, Resolution of Sidebands in a Semiconductor Laser Frequency Modulated by Ultrasonic Waves; IEEE Journal of Quantum Electronics, vol. Q3-6, No. 6, June 1970, pp. 352-355. Although capable of providing a limited degree of frequency modulation, such acousto-optic modulators have proven to be incompatible with semiconductor device planar processing techniques. That is, a separate processing step is required to externally attach the quartz transducer or like acoustical element to a lateral surface of the diode laser. Moreover, this separate processing renders such devices incapable of being monolithically integrated with other integrated circuit elements.
Perhaps more significantly, conventional acousto-optie modulation structures have not been shown to be capable of providing the type of high-frequency modulation required by various advanced optical communications and signal processing systems. Moreover, the modulation index associated with the modulated optical output of such conventional structures is generally less than would be desired to facilitate heterodyne detection and demodulation. Existing acousto-optie modulation structures are also not disposed to provide phase modulation in the absence of appreciable frequency modulation, thereby further restricting their utility within advanced optical systems.